4-Hydroxy-2-nonenal (HNE), one of the cytotoxic aldehydes produced by lipid peroxidation, is noted for its ability to modify proteins at certain amino acid residues. The reaction of HNE and lysine derivatives was studied in details so as to gain some understanding of the chemistry of modification processes in the protein. Among the complex mixture of reaction products was a fluorescent component (compound C) whose spectral characteristics were very similar to those of lipofuscin. Thus, the structural elucidation of this fluorescent compound was of interest for the knowledge of the chemical nature of lipofuscin. Various spectroscopic and chemical data obtained thus far revealed that compound C was derived by oxidation of a 1-2 adduct of HNE and the lysine derivative. One of the lysine units was attached intact to a chromophoric group consisting of a conjugated triene or cyclic diene involving the amino group of the other lysine unit in the molecule.